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A very belligerent planet. Mars. Named after the Roman god of war, Mars appears as a fiery battle-stained planet. Out of the seven planets (excluding Earth), Mars seems to be the most habitable planet. Although much smaller than Earth, Mars has an atmosphere (though thinner), ice caps (though mainly dry ice, or frozen carbon dioxide), and evidence that water once flowed on its surface. Astronomers have yet to find life on Mars, but sci-fi authors have long contemplated the possibility of life on Mars in the present or future (e.g. The Martian Chronicles). Don’t let the name fool you. Mars, the god of war, leaves no one alive while Venus, the goddess of love, often has a soft heart. In the solar system, Venus is a deadly planet while Mars is relatively benign.

The fourth planet from the Sun and the last terrestrial planet, Mars, the “Red Planet,” is actually the second smallest planet and the most explored aside from Earth. Distinguishable by its red color from an abundance of iron oxide, Mars has a thin atmosphere blanketing a surface filled with impact craters, volcanoes, valleys, deserts, and polar ice caps. Mars is also home to Olympus Mons, the tallest mountain in the solar system, and Valles Marineris, a large crack on Mars’ surface and one of the largest canyons. Dark patches on its surface suggest presence of large quantities of liquid water in the past. Though similar to Earth in surface features and climate, Mars is only about half the size of Earth, with 15% of Earth’s volume and 11% of mass. Because of its similar axial tilt, Mars has Earth-like seasons, though with a colder climate. Barren, Mars has the largest dust storms in the solar system. Mars is bigger than Mercury but less dense because of an iron sulfide core composed of lighter elements. The core is surrounded by a silicate mantle and the mantle by a thick crust. With no evidence of a global magnetic field, Mars, however, has some magnetized crust that has reversed polarity like Earth’s ocean floors. Mars’ geological history is split into three periods: Noachian period (4.5 – 3.5 billion years ago, oldest surfaces with impact craters and extensive flooding of water), Hesperian period (3.5 to 2.9 – 3.3 billion years ago, extensive lava plains), Amazonian period (2.9 – 3.3 billion years ago to present, few impact craters, Olympus Mons forms, lava flows). On Martian Soil, light silica-rich streaks appear on steep slopes, perhaps the dark underlying layers of soil exposed after dust avalanches. As Earth speeds Mars, Mars appears to move in a retrograde motion, or backwards with respect to the stars. Mars has two moons: Phobos and Deimos.

PHOBOS and DEIMOS

Phobos and Deimos

Mars’ two moons look more like asteroids than Earth’s moon. Named for Mars’ sons and attendants in battle, Phobos and Deimos mean “fear” and “panic,” respectively. Because of their low albedo, carbonaceous chondrite composition, and irregular shapes, scientists have proposed the capture theory. Stray asteroids from the Trojan belt may have been pulled into Mars orbit. Phobos is below synchronous position, so Mars’ tidal forces will eventually cause Phobos to crash into Mars’ surface, either forming an impact crater or a dust ring in 50 million years. On the contrary, Deimos is outside synchronous position, so the moon is slowly spiraling away from Mars. In about 50 million years, Mars may have no moons!

What comes to mind when you think about Mercury? Perhaps the poisonous silvery liquid in old thermometers or the Roman counterpart of the Greek messenger god Hermes? The first planet of the solar system? The Moon’s look-alike? Or maybe… all of the above!

The innermost and terrestrial planet, Mercury is closest to the Sun and feels more of the Sun’s gravity than any other planet. Discovered as early as the 14th century BC, Mercury is one of the ancient planets. Mercury, like the speedy Roman messenger god, is also the speediest planet, traveling on the most elliptical (eccentric) orbit of the 8 planets. As the smallest planet, Mercury has the weakest gravity and no moons. With its heavily cratered surface, Mercury easily looks similar to Earth’s Moon and has been geologically inactive for billions of years. Its negligible atmosphere offers little resistance and protections against onslaughts of meteors and asteroids. Surprisingly, Mercury is not the hottest planet of the solar system even though it is closest to the Sun, because its very thin atmosphere cannot trap much heat. Moreover, Mercury has a low albedo (not very reflective) since its atmosphere mostly absorbs rather than reflects light from the Sun. Consequently, the Sun’s light obscures the already dim Mercury from our view, but individuals usually observe Mercury at dawn or dusk during minimal sunlight— hence, the terms morning star and evening star. Mercury’s largest surface feature is the 4,000-mile Caloris Basin, one of ~15 impact craters. Near the Caloris Basin is a region of hilly terrain called the “Weird Terrain.” Mercury has large ridges up to several hundred kilometers high on its surface. Other features are smooth plains and compression folds (rupes). Mercury’s core is large and rich in iron, since its gravity is not as strong as larger planets to compress it. It is 70% metallic and 30% silicate material. Around the now-believed-to-be molten core is the 500-700 km thick mantle and a 100-300 km think crust. In addition, due to its slow rotational period and small size, Mercury has a significant magnetic field, about 1.1% as strong as Earth’s. Like, Venus, Mercury appears in phases when observing from Earth. On average, Mercury is the closest planet to Earth at 1.08 AU (Earth to Venus is 1.3 AU).

HOW DID MERCURY FORM? – 3 Hypotheses

Mercury was struck by a planetesimal 1/6 the mass of the planet and several kilometers across. The collision destroyed most of the crust and mantle, leaving a large core.

Mercury was formed by the solar nebula (a gaseous cloud from which the Sun and planets formed by condensation) before the Sun’s energy output stabilized. Temperatures during the formation could have been as low as 2,500 K – 3,000 K or as high as 10,000 K. Originally twice the size as it is now, the protosun’s contraction high temperatures vaporized the outer layers of rock on Mercury.

The solar nebula caused drag on materials accreting into Mercury, so that light elements left and heavier elements remained.

* MESSENGER found higher levels of potassium and sulfur than previously thought on Mercury’s surface. This means hypotheses 1 and 2 are unlikely, thus favoring the third hypothesis.

August 10, 1990: Magellan enters orbit around Venus. Over the next four years, it maps 98 percent of the planet’s surface.

October 6, 1990: The U.S. – European Ulysses spacecraft launches a mission to study the Sun and its poles.

October 29, 1991: En route to Jupiter, Galileo makes the first flyby of an asteroid when it passes by Gaspra.

August 10, 1992: The U.S. – French ocean-monitoring satellite Topex/ Poseidon launches.

August 28, 1993: Galileo flies by a second asteroid, Ida, on its way to Jupiter.

December 2, 1993: Shuttle astronauts take a spacewalk to install JPL’s Wide-Field and Planetary Camera 2 in the Hubble Space telescope, compensating for a flaw in the telescope’s main mirror. The instrument allows Hubble to capture remarkable images of galaxies, nebula, planets, and many other celestial objects.

April 9, 1994: A decade after the first shuttle radar imaging mission, the third in the series launches. A JPL instrument is combined with a German-Italian radar system.

February 12, 1997: JPL teams with a Japanese spacecraft launched under the Space Very Long Baseline Interferometry program to make radio observations of the distant Universe.

July 4, 1997: Mars Pathfinder lands, delivering the first mobile rover to another planet, By the final data transmission on September 27, the mission returns 2.3 billion bits of information, including more than 16,500 lander images and 550 rover images.

September 12, 1997: Mars Global Surveyor enters orbit.

October 15, 1997: Cassini launches to travel 6-1/2 years to Saturn, where the European-built Huygens probe will descend to the surface of the shrouded moon Titan.

February 17, 1998: Voyager 1 passes another spacecraft to become the most distant human-made object in space.

October 24, 1998: Deep Space 1 launches on a mission to flight-test advanced technologies, including an ion propulsion system.

February 7, 1999: Stardust launches on a mission to fly past a comet and return samples of comet and interstellar dust to Earth.

June 19, 1999: The Quick Scatterometer satellite launches into Earth orbit to study near-surface ocean winds around the globe.

December 14, 2009: Wide-field Infrared Survey Explorer launches. It will scan the sky in infrared light, creating a vast catalog of celestial objects.

May 20, 2010: The Mars Exploration Rover project passes a historic longevity record: “Opportunity” rover surpasses the duration record set by the Viking 1 lander of 6 years and 116 days operating on the Martian surface.

September 27, 2010: Cassini begins its second extended mission, named the Cassini Solstice Mission.

November 1, 2010: The giant 70-meter (230-foot) antenna at the Goldstone Deep Space Communications goes back on line tracking deep space missions after a seven-month upgrade.

November 4, 2010: Deep Impact-EPOXI flies by comet Hartley 2.

December 25, 2010: Mars Odyssey becomes the longest-serving spacecraft at Mars – 3,340 days in orbit.

August 27, 1962: Mariner 2 launches and conducts the first flyby of another planet when it visits Venus on December 14.

July 28, 1964: Ranger 7 launches and executes an intentional crash-landing into the Moon on July 31. As it closes in, it sends back more than 4,000 pictures of the lunar surface.

November 28, 1964: Mariner 4 launches with a destination of Mars.

February 17, 1965: Ranger 8 launches and impacts the Moon in Mare Tranquillitatis three days later. This location will become the landing spot for the Apollo 11 astronauts 4-1/2 years later.

March 21, 1965: Ranger 9 launches and three days later impacts the Moon in the 108-kilometer-diameter (67-mile) crater Alphonsus, sending back more than 5,800 images.

July 14, 1965: After an eight-month voyage to Mars, Mariner 4 makes the first flyby of the red planet. The spacecraft radios back the first close-up photos of another planet.

May 30, 1966: Surveyor 1 launches. On June 2, it becomes the first U.S. spacecraft to make a soft landing on the moon.

April 17, 1967: Surveyor 3 launches, lading on the Moon on April 20. Two and a half years later, the Apollo 12 astronauts will land nearby and photograph the Surveyor 3 site.

June 14, 1967: Mariner 5 launches and flies by Venus on October 19.

September 8, 1967: Surveyor 5 launches and lands on the Moon September 11.

November 7, 1967: Surveyor 6 launches and soft-lands on November 10. The lunar mission runs until December 14.

January 7, 1968: Surveyor 7, the last of the Surveyor series, launches and soft-lands on the Moon on November 9. Overall, the Surveyors acquire 90,000 images from five sites on the Moon.

February 24, 1969: Mariner 6 launches. A month later, on March 27, Mariner 7 launches. They complete the first dual mission to Mars with flybys on July 3 and August 5.

May 30, 1971: Mariner 9 launches and reaches Mars on November 13, becoming the first spacecraft to orbit another planet. The craft operates for nearly a year around Mars.

November 3, 1973: Mariner 10 launches on a mission to Mercury and Venus, the first craft designed to visit two planets.

February 5, 1974: Using gravity-assist for the first time, Mariner 10 swings by Venus to borrow the planet’s gravity to propel it on to Mercury, which it passes on March 29. On September 21, it flies past Mercury again.

March 16, 1975: Mariner 19 flies by Mercury a third time.

August 20, 1975: Viking 1 launches an orbiter and lander toward Mars. On September 9, Viking 2 launches a similar orbiter- lander pair.

June 19, 1976: Viking 1 arrives in orbit at Mars. On July 20, its lander becomes the first craft to soft-land on another planet. On August 7, Viking 2 arrives in orbit, and its lander touches down on September 3.

August 20, 1977: Voyager 2 launches, followed by the launch of Voyager 1 on September 5.

June 26, 1978: The experimental Seasat satellite launches to test four instruments that use radar to study Earth and its seas. The satellite collected more ocean topography data than the previous 100 years of shipboard research.

March 5, 1979: Voyager 1 makes its closest approach to Jupiter. On July 9, Voyager 2 flies by the giant planet. Together, the Voyagers take more than 22,000 images of Jupiter and its moons.

November 12, 1980: Voyager 1 flies by Saturn.

August 25, 1981: Voyager 3 flies by Saturn.

October 6, 1981: The Solar Mesosphere Explorer launches to study processes that create and destroy ozone in Earth’s upper atmosphere.

November 12, 1981: The first in a series of radar imagers is launched on the space shuttle.

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Astronomy: To Infinity and Beyond! Welcome to "The Cosmos." I will take you on a journey through our solar system, galaxy, and the Universe! You will be updated with current events in astronomy. Please click on the picture above to visit my blog on poetry, writings, and musings!

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References for photos used from websites can be found under the "References" page. Photo credit: news sites (reference included in post), NASA (most images used), and Google (for artists' view of objects unable to be photographed).